This disclosure relates generally to a method and system for regenerating and/or desulfating NOx adsorbers and/or regenerating particulate filters.
In general, diesel engines generally emit less nitrogen oxides (NOx) than a gasoline engine under most conditions, but because diesel engines mostly or exclusively operate on a high air to fuel ratio, the chemistry of the exhaust gas does not favor NOx reduction, because of the excess of oxidizing species. Thus, the reduction of nitrogen oxides, e.g., nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), in exhaust gas is a widely addressed problem as a result of environmental concerns and mandated government emissions regulations, particularly in the transportation industry. One proposed solution is the use of a three-way conversion catalyst, which can be employed to treat the exhaust gases. Such three-way conversion catalysts, contain precious metals such as platinum, palladium, and rhodium, and can promote the oxidation of unburned hydrocarbons and carbon monoxide, and the reduction of nitrogen oxides in exhaust gas provided that the engine is operated around a balanced stoichiometry for combustion (also referred to as xe2x80x9ccombustion stoichiometryxe2x80x9d). The balanced combustion stoichiometry is typically at an air to fuel ratio between about 14.4 to about 14.7.
However, fuel economy and global carbon dioxide emission concerns have made engine operation under lean-burn conditions desirable in order to realize a benefit in fuel economy. Under such lean-burn conditions, the air-to-fuel ratio may be greater than the balanced combustion stoichiometry, i.e., greater than about 14.7 and may be between about 19 to about 35. When lean-burn conditions are employed, three-way conversion catalysts are generally efficient in oxidizing the unburned hydrocarbons and carbon monoxides, but are generally inefficient in the reduction of nitrogen oxides.
One approach for treating nitrogen oxides in exhaust gases is to incorporate a NOx adsorber, also referred to as a xe2x80x9clean-NOx trap,xe2x80x9d in the exhaust lines. The NOx adsorber promotes the catalytic oxidation of nitrogen oxides by catalytic metal components effective for such oxidation, such as precious metals. The formation of NO2 is generally followed by the formation of a nitrate when the NO2 is adsorbed onto the catalyst surface. The NO2 is thus xe2x80x9ctrappedxe2x80x9d, i.e., stored, on the catalyst surface in the nitrate form. The system can be periodically operated under fuel-rich combustion to regenerate the NOx adsorber. During this period of fuel-rich combustion, the absence of oxygen and the presence of a reducing agent promote the release and subsequent reduction of the stored nitrogen oxides. However, this period of fuel-rich combustion may also result in a significant fuel penalty.
As previously mentioned, exhaust gas streams can further comprise particulate matter such as carbon-containing particles or soot. A particulate filter, commonly used with compression ignition engines, can be used to prevent the carbon particles or the soot from exiting a tailpipe. The particulate filter may be a stand-alone device separate and distinct from devices employing catalytic elements for removing undesirable NOx gaseous components. Carbon particles can be trapped in the particulate filter and then periodically burned to regenerate the filter.
Regeneration of particulate filters can be accomplished by the use of auxiliary devices such as a burner or other heating element. For example, an air-fuel nozzle and an ignition device can be used and operated, when desired, to heat the exhaust gases and the particulate filter to a combustion temperature of the trapped particulate matter. In this manner, the trapped particulate matter can be burned from the filter surfaces to permit a continuous flow of the exhaust gases. Alternatively, an electric heater can be used to generate the heat to initiate the combustion of the trapped particulates. However, these approaches are limited by their energy efficiency, durability, and cost.
Disclosed herein is a system for regenerating and/or desulfating a NOx adsorber and/or a system for regenerating a particulate filter. The system comprises regeneration system comprising an exhaust conduit in fluid communication with an exhaust fluid from an engine, wherein the exhaust conduit comprises a first oxidation catalyst, a NOx adsorber, and a second oxidation catalyst coupled to a particulate filter; a fuel source in fluid communication with a reformer, wherein the reformer is adapted to generate a hydrogen and carbon monoxide containing fluid from a fuel supplied by the fuel source; a regeneration conduit in fluid communication with the exhaust conduit and the reformer; and valve means disposed in the regeneration conduit for selectively controlling and directing the hydrogen and carbon monoxide containing fluid from the reformer to the first oxidation catalyst, the coupled second oxidation catalyst and particulate filter, the NOx adsorber, or a combination thereof.
In accordance with another embodiment, a regeneration system comprises an exhaust conduit in fluid communication with an exhaust fluid from an engine, wherein the exhaust conduit comprises a first oxidation catalyst, a second oxidation catalyst coupled to a NOx adsorber, and a third oxidation catalyst coupled to a particulate filter; a fuel source in fluid communication with a reformer, wherein the reformer is adapted to generate a hydrogen and carbon monoxide containing fluid from a fuel supplied by the fuel source; a regeneration conduit in fluid communication with the exhaust conduit and the reformer; and valve means disposed in the regeneration conduit for selectively controlling and directing the hydrogen and carbon dioxide containing fluid from the reformer to the first oxidation catalyst, the second oxidation catalyst coupled to the NOx adsorber, and the third oxidation catalyst coupled to the particulate filter, or a combination thereof.
A process for regenerating and desulfating a NOx adsorber and/or a regenerating a particulate filter comprises periodically supplying a fuel to a reformer; converting said fuel to a hydrogen and carbon monoxide containing fluid; selectively feeding the hydrogen and carbon monoxide containing fluid into an oxidation catalyst or into a NOx adsorber, or into the oxidation catalyst and the NOx adsorber catalyst; and generating an exotherm in the oxidation catalyst and heating an exhaust fluid passing therethrough to a temperature effective to regenerate a particulate filter disposed downstream from the oxidation catalyst or reducing nitrogen oxides adsorbed by the NOx adsorber or generating the exotherm in the oxidation catalyst and heating the exhaust fluid to the temperature effective to regenerate the particulate filter downstream from the oxidation catalyst and reducing the nitrogen oxides trapped by the NOx adsorber.
The above-described and other features will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.